This is an embarrassing novice question ... I got a mountain bike with a fork that has low and high speed compression damping settings. This is the first time I am riding a bike with an air fork. I do not have experience with them. I cannot notice any difference in the fork's behaviour when going from minimum to maximum damping or vice versa. In contrast, I can notice a difference in the rebound setting. Is this normal? How can I determine if it is working correctly?
Try taking all the air out so you can feel just the damper’s resistance. Low speed compression will be noticeable when doing body movements such as preloading, weight shifting, or braking. High speed compression is for events such as hitting big drops and large square edge impacts. The easiest way to test that is to smash into a square curb (perhaps with a bit of air to prevent fork damage).
Remember that the “speed” here is shaft speed, not bike speed. Shaft speed is how fast the damper shaft is moving through the oil column. Smashing a curb at walking speed is a high shaft speed event, just as pumping through a compression at 40km/h is a low shaft speed event.
I would pump your fork up to the manufacturers suggested air pressure for your riding weight. Then determine if this gives you the desired sag. Sag is the distance into your travel that your body and gear weight cause on the suspension, and is an important baseline to get correct first before any other adjustments. In an air fork, sag is a function of the air pressure and adjusting air pressure changes the sag. Your riding weight is with the gear, including backpack or hydration pack you would normally ride with. Don't worry if this stuff varies between rides, just generally be at your riding weight when setting sag. The fork will have a sag indicator o-ring usually on the left, air spring side. To set sag--which should be 25-30% of the full travel of the fork--sit on the saddle and assume your normal riding position. Leaning up on a wall or tree or have another person assist, bob up and down on the fork a few times while holding the brake. This breaks any stiction present. Back to your riding position and take a minute for the suspension to settle in. Carefully slide the sag indicator o ring down the stanchion until it touches the seal. Now you must carefully dismount the bike while being careful to not activate the suspension which will cause the sag measurement to be inaccurate. A bucket or step ladder placed beside the bike helps to be able to get off the bike. When you're off the bike measure the distance between the indicator ring and the oil seal. For 100mm travel forks it should be 25-30mm. For 120mm travel forks the measurement is 30-35mm. If your travel is different, simply multiply the the forks total travel by 25% then 30% to obtain the suggested sag measurement range. To adjust the amount of sag you need to adjust your air pressure. If you are getting under 25% sag, you'll need to decrease air pressure in 5 psi increments until you achieve 25-30%. Air pressure is adjusted with a special high pressure shock pump attached to the schraeder valve on the top of the left fork leg. (Left/right is defined from the rider's perspective in the saddle). To remove air, attach pump and use the microadjust button). If the sag measurement is too high--over 30% of travel--air should be added to the air spring, also in 5-10 psi increments. I usually leave my pump attached to the schraeder valve while I set my sag. This is for convenience and that I kind of know what numbers I'm shooting for. Some might think it will lead to inaccuracy since there is an additional air volume held within the pump and hose. However the pressure reading is that volume of air in the fork and when you disconnect the hose air remains inside the fork and what you hear is air escaping from the hose and pump. You do need to be off the bike to determine the proper pressure reading, then remount to set sag.
Fox recommends an initial rebound setting of 1/2 the number of total clicks out from fully in. Thus if you have 12 total clicks of adjustment, turn the rebound adjuster fully clockwise (do not force the dial past it's stopping point. This can damage the Internals or jam the rebound needle making it difficult to adjust). So fully in and then back out 6 clicks (determine the total amount of clicks you have by counting them as you unwind the adjuster out from fully in. Generally it's 12 to 14 clicks). This is your baseline rebound setting and can be altered as you ride.
Low speed compression controls the damping of smaller, low shaft speed hits and rider movement like controlling peddle bob. You have, again, a number of clicks of adjustment and a good starting point is 1/2 the number of total clicks. You will want to set all these parameters with any 2 or 3 position ride control lever in full open (counter clockwise) position. In other words, you may have a lever on top of the right fork leg that has "open, medium, firm" or "climb-trail-descend" and you'll want to make adjustments in open or descend mode.
High speed compression is high shaft speeds encountered with larger square edged hits or big drops. The adjuster on the fork is labled HSC, and again the manufacturer will indicate in the manual or sometimes on a sticker on the fork, what a good baseline setting will be for your weight.
To make adjustments out on the trail, I suggest using a method called stacking. This is making adjustments to one parameter at a time, riding over the same section of trail that has a variety of bumps and texture, and determining subjectively how it feels. After your baselines are set, ride the selected trail section, pick an adjustment, and make two clicks either way. I usually go out/counter clockwise with my adjustments. So 2 clicks out on 1 adjuster and ride the trail section again. Determine better or worse ride quality. If better go 2 more clicks out (or two clicks in the same direction that saw improvement in the ride), ride the trail section again, better or worse? When it comes to the point where the ride wasn't as good. Go back one click and see how that feels. It's a good idea to write down your settings for a quicker set up after maintenance or someone else was riding and made adjustments.
I've linked to some helpful videos explaining suspension set-up. The "Tuesday Tune" by Vorsoprung is one of my favorite series on suspension since they get more in depth as to what is going on inside the shock or fork and explain the mechanics behind them. Also, they aren't shy about offering an opinion whether good or bad about a product or issue. This is refreshing in an industry that often has too much influence on what is said about their products.